Investigation of persulfate production on BDD anode by understanding the impact of water concentration

Cathodic hydrogen production by simultaneous oxidation of methyl red and 2,4-dichlorophenoxyacetate in aqueous solutions using PbO2, Sb-doped SnO2and Si/BDD anodes. Part 2: hydrogen production

“…For this case, Tafel analysis showed that chemical and/or electrochemical processes taken place involving electron transfer processes in some particular sequence, being more evident when 2,4-DNa was present in the electrolytic solution. This fact highlights the participation of both type of oxidizing species (cOH as well as SO 38 as well as each one of the oxidizing species can participate, directly, indirectly, or collaboratively to inuence the OER or to intervene in the oxidation of the organic pollutants. 32 These last behaviors are more evident when H 2 production is analyzed in the current second part of this study.…”

“…6b. In our previous work, 35 it was proposed that by using H 2 SO 4 as supporting electrolyte the oxygen production through cOH radical physically adsorbed on BDD surface, is limited by the formation of sulphate-oxidizing species (SO 4 À c and S 2 O 8 2À ), 35,36,38 BDD(cOH) + SO 4 2À / BDD(SO 4 À c) + OH À Furthermore, the oxygen production rate r(O 2 ) at BDD anode in H 2 SO 4 (0.0004 L min À1 ) is also the half-value of the estimated value than in HClO 4 (0.0008 L min À1 ), therefore, it is reasonable to suppose that the hydrogen production rate should be also minor in H 2 SO 4 , such as is reported in Table 2.…”

Cathodic hydrogen production by simultaneous oxidation of methyl red and 2,4-dichlorophenoxyacetate aqueous solutions using Pb/PbO2, Ti/Sb-doped SnO2and Si/BDD anodes. Part 1: electrochemical oxidation

“…An important feature to be highlighted about this model, is the fact that the SO 4 À c are formed on BDD anodes only in a sulfate medium (H 2 SO 4 or Na 2 SO 4 ). 40,50 It was veried in 0.5 M NaClO 4 solution (Fig. 12) where both the whole diminish of the UV spectra and the absence of UV bands around 250 nm are related to a combustion process of the organic compound on BDD anodes, 51 in absence of sulfate-oxidizing species.…”

“…As mentioned, the overpotential for OER and to the adsorption enthalpy of hydroxyl species on the anode surface, i.e., for a given anode material, the higher is the O 2 overvoltage the higher is its oxidation power, are the main factors that determinates the electrooxidation capacity of non-active anodes. However, it was recently shown that BDD electrodes has also the capacity to produce S 2 O 8 2À and SO 4 À c when sulfate is used as supporting electrolyte [38][39][40] according to the reaction scheme (14): In the case of the Pb/PbO 2 and Ti/Sb-doped SnO 2 anodes, even when very similar overpotential for oxygen evolution (about 1.9 V/RHE vs. 2.1 V/RHE) were registered, Ti/Sb-doped SnO 2 electrode exhibits slower MR discoloration rate and also lower current efficiency. This behavior is related to changes on the SnO 2 surface structure under anodic polarization conditions; for instance, the hydration of the -Sn]O bond is achieved, and that the R pollutant is then adsorbed on the hydrated surface and, subsequently, oxidized by adsorbed cOH.…”

“…However, the S 2 O 8 2À production could be only associated to the presence of hydroxyl radicals on the surface of anodes. 40,54 Certainly, if the kinetic data for producing hydroxyl radicals on Pb/PbO 2 and on Ti/Sb-doped SnO 2 (Table 1) are compared, then, PbO 2 is more efficient that SnO 2 , which also explain its electroactivity to favor the conversion from 2,4-DNa to 2,4-DP. This result is in agreement with the apparent rate constants above quoted.…”